Acidic monomers, e.g. carboxylic, sulfonic, phosphonic acids and dihydrogen phosphates, are used in various kinds of dental materials, such as adhesives, self-adhesive resin cements or compomers. Self-etch adhesives (SEAs) are used to achieve a strong bond between restorative composites and the dental hard tissues (dentin and enamel). SEAs are aqueous solutions containing acidic monomers, cross-linking dimethacrylates, such as Bis-GMA (an addition product of methacrylic acid and bisphenol-A diglycidylether) or UDMA (an addition product of 2-hydroxyethyl methacrylate (HEMA) and 2,2,4-trimethylhexamethylenediisocyanate), monofunctional comonomers (e.g. HEMA), initiators and additives. The acidic monomer is the key component of SEAs as it is responsible for the etching of the dental hard tissues.
Self-adhesive resin cements (SARCs) adhere to both dentin and enamel without using an additional adhesive. Because of their simplicity of application, such materials have gained in popularity among dentists. SARCs comprise bulky crosslinking monomers (e.g., BisGMA or UDMA), diluents, such as triethylene glycol dimethacrylate (TEGDMA), an acidic monomer, different kinds of fillers, initiators and additives. In SARCs, acidic monomers are able to partially demineralize the dental hard tissues as well as to form a strong chemical adhesion with hydroxyapatite. Additionally, they can react with the filler to release fluoride ions.
Monomers bearing an acidic group (e.g. a phosphonic acid group) exhibit a high reactivity in free-radical polymerization. This phenomenon has been attributed to the ability of the acidic group to form strong hydrogen bonds. Acidic monomers commonly used in dental materials are, for example, 10-(methacryloyloxy)decyl dihydrogen phosphate (MDP), 2-(methacryloyloxy)ethyl dihydrogen phosphate (MEP), 4-[4-(methacryloyloxy)ethoxycarbonyl]phthalic acid (4-MET) or 2-[4-(dihydroxyphosphoryl)-2-oxa-butyl]acrylate (EAEPA).
EP 1 057 468 A1 and corresponding U.S. Pat. No. 6,512,068, which is hereby incorporated by reference, disclose dental adhesives containing polymerizable dihydrogen phosphates such as MDP. The use of polymerizable phosphonic acids in dental materials is, for example, described in EP 1 374 829 A1 and corresponding U.S. Pat. No. 6,900,251, which is hereby incorporated by reference, EP 1 169 996 A1 and corresponding U.S. Pat. No. 6,902,608, which is hereby incorporated by reference and WO 02/02057 A1.
The performance of SEAs and SARCs can be improved by incorporating acidic monomers exhibiting strong chelating properties. The use of β-ketophosphonic (EP 2 816 049 A1 and corresponding US 2016145277, which is hereby incorporated by reference,) and diphosphonic acids (WO 2004/060327 A1 and corresponding U.S. Pat. No. 8,404,144, which is hereby incorporated by reference, EP 2 755 624 A1 and corresponding U.S. Pat. No. 9,040,602, which is hereby incorporated by reference) in SEAs has been shown to result in a strong bond between a dental composite and the dental hard tissues.
Phosphonic acids bearing a urea group (EP 2 823 801 A1 and corresponding US 2016151249, which is hereby incorporated by reference,) were also able to significantly improve the adhesion of dental materials.
JP 2012-006880 A relates to dental adhesives containing hybrid acidic monomers bearing both a phosphonic and a carboxylic acid groups. US 2010/0076157 A1, which is hereby incorporated by reference, discloses the preparation of polymerizable dihydrogen phosphates comprising a carboxylic acid group and their use in dental materials.